1. Field of the Invention
The present invention relates to a data transmission device suitable for data transmission in a communication environment in which code errors easily occur.
2. Background Art
In data transmission using a communication line in which code errors easily occur, a data lack or a miss receiving due to code errors often occurs in a receiver device and this deteriorates quality of receiving. In order to overcome this problem, technical means such as a retransmission control method or an error correction decoding is used. These technical means are disclosed in detail, for example, in a printed material written by Shu Lin and Daniel J. Costello under the heading of "Error Control Coding Fundamentals and Applications" Prentice Hall 1983.
A retransmission control method is a method in which a transmitter device transmits duplicate data in response to a receiving error report from a receiver device. The retransmission control method is significantly effective to a data communication in which many code errors occur at a burst. However, the retransmission control method requires a reverse channel through which the receiver device sends the receiving error report to the transmitter device, and therefore the method cannot be used in data transmission devices having no reverse channel.
Error correction coding is a method in which a receiver device corrects code errors of received data and therefore the method does not require any reverse channel. However, there is a case in which many code errors occur at a burst in a communication line and the code errors exceed the error correction ability. In such a case, a part of the code errors are not corrected in the error correction decoding and some code errors are remained in the decoded data. In order to reduce the remaining error, use of interleave is expected. However, when using interleave in data transmission, error correction ability corresponding to the number of code errors which are expected to be included in a section on which the interleave is carried out. Therefore, a very high redundancy is necessary for the error correction coding. Furthermore, when a heavy burst error exceeding the error correction ability occurs in a part of a section on which the interleave is carried out, it is impossible to correctly decode the all received data even if the other part of the section includes no code error.
In order to protect transmitted data from code errors without using reverse channel, a method in which a data and a error detection code added to the data are repeatedly transmitted at discrete plural times is expected as an effective method. In this method, the receiver device receives same data at plural discrete times, and the receiver device carries out error detection on the all received data, which are the same data if having no code error, and decodes one of the received data from which no code error is detected.
The conventional data transmission methods are as described above. In the conventional art, it is quite difficult to provide an effective data transmission in which transmitted data can be protected from burst code errors and random code errors without use of reverse channel. The reason is more specifically described as follows:
In order to protect transmitted data from burst code errors, it may be effective to use the above-described method to transmit same data at plural discrete times.
When same data are transmitted at plural times according to the method, a part of the data may be transmitted to the receiver device with no code error. Therefore, if such a data having no code error can be transmitted and the data thus transmitted is decoded, it is possible to prevent that the transmission quality is degraded by a data lacking due to code errors.
In some cases, however, random code errors occur in the all data which are discretely transmitted at plural times according to the above method. In such cases, the receiver device can detect the code errors of the all received data because the all data have the error detection codes. However, the receiver device cannot correct the code errors. Therefore, in this case, a data having no code error is not obtained in the receiver device at all even if each received data include little random code error.
In order to overcome this problem, an method in which error correction coding is carried out on all data discretely transmitted at plural times and the errors of the data are corrected in the receiver device, is expected. However, if this method is used, a very high redundancy is required to obtain enough error correction ability. The use of the method to duplicately transmit same data deteriorates the transmission efficiency to some extent. However, the use of error correction coding deteriorates the transmission efficiency to more heavy extent, and therefore the transmission efficiency is significantly deteriorated.
Furthermore, this method has a problem that the control of the redundancy requires control of frequency at which same data are transmitted and there is no method except for the method controlling frequency, and therefore the method lacks flexibility of redundancy adjustment in that the redundancy can be adjusted to only integrals of that of the original data.